High-frequency and high-speed amplifiers are generally either cascaded or distributed. A cascaded lumped element amplifier can be implemented using narrowband matching elements or broadband matching elements, examples of which are shown in the circuit 100 of FIG. 1A. For cascaded amplifiers, the overall voltage gain, G, is the product of the gain of individual stages. In other words, G≈Π between the input 102 and the output 104 for  stages. Each stage includes a transistor 106 with the transistor gate connected to either the input 102 or the output of a previous stage drain 110. Narrowband matching as in the circuit 100 of FIG. 1A typically allows the highest gain for a given device. A resonant network matches the impedance at the gate, source, or drain of the device to the source and load impedance. These input and output matching networks are well suited to integrated implementations with finite quality factor passives and can alternatively optimize the noise, linearity, or output power of the amplifier stage. However, a drawback of narrowband matching is that the input and output matching are only acceptable over a narrow bandwidth. Additionally, the bandwidth of the amplifier response generally reduces through multiple stages of amplification. For W-band applications, this technique has been preferred to achieve high gain, low noise amplifiers in any solid-state circuit technology.
Feedback techniques such as the shunt resistive feedback circuit 130 of FIG. 1B is capable of wideband matching and gain response. However, resistive shunt feedback inherently reduces the voltage gain of each stage and provides matching determined by the shunt feedback resistance. Additionally, shunt feedback tends to result in higher noise figure due to the addition of the shunt feedback resistance.
An alternative approach for broadband matching and gain is distributed amplification, often referred to as traveling wave amplification, an example of which is shown in the circuit 160 of FIG. 1C. In the example of FIG. 1C, the circuit 160 includes a series of stages that each include an active device 162, such as for example a field effect transistor (FET) or a bipolar junction transistor (BJT). An input transmission line 164 runs from an input 166 to a first impedance termination 170 and an output transmission line 172 runs from a second impedance termination 174 to an output 176. The active device 162 of each stage has its gate or base 180 linked to the input transmission line 164, its drain or collector 182 to the output transmission line 172, and its source or emitter 184 terminated. Other configurations of distributed amplifiers are also possible. In general, a distributed amplifier 160 absorbs the capacitances of the active devices 162 into an artificial transmission line along the input transmission line 164 and output transmission line 172. The input traveling wave propagates along the input transmission line 164, excites the active devices 162 of each stage along the input transmission line 164, and is absorbed into the first impedance termination 170 at the end of the input transmission line 164. In response, two output traveling waves are generated at the drain 182 of each stage; a forward traveling wave that is constructively amplified across multiple stages and an undesired backward traveling wave that is lost in the second termination impedance 174. Combining the output forward traveling wave at each stage results in a lower overall voltage gain than in a cascaded amplifier configuration. The gains of each stage are summed rather than multiplied, so G≈Σ for  stages.
Distributed amplifiers are generally limited by inherent quality factor losses along the gate and drain artificial transmission lines, so typically no more than six to eight stages can be implemented in a conventional distributed amplifier. The achievable gain therefore remains relatively low. However, a distributed amplifier does advantageously provide a desirable broadband gain response accompanying the broadband input and output matching. At millimeter wave frequencies, distributed amplifiers were originally demonstrated with indium-phosphide (InP) and gallium-arsenide (GaAs) technologies. The advance of silicon into millimeter wave regimes has more recently pushed distributed designs into silicon technologies using both n-type metal-oxide-semiconductor (NMOS) devices.